Previous research indicates that total sleep deprivation may elicit a number of physiological and psychological deficits including mood disturbance and autonomic imbalance. Little is known regarding the effects of a single night of moderate sleep loss, a much more common occurrence. The aim of this study was to determine the effects of acute moderate sleep deprivation (AMSD) on resting heart rate variability (HRV) and mood, as well as heart rate (HR), oxygen consumption (VO2), peripheral blood lactate and subjects’ ratings of perceived exertion (RPE) during steady state exercise. Thirteen healthy male subjects (age: 20+0.25 years; peak VO2:=52.0+ 2.7 ml/kg/min) gave informed, written consent and underwent initial testing to determine peak VO2 on a calibrated cycle ergometer. Following the AMSD (2-4 hours of sleep) or rested (8-10 hours of sleep) state, subjects completed submaximal exercise tests. Prior to the sub-maximal tests, subjects were administered the Profile of Mood States (POMS) inventory and resting heart rate variability (HRV) was recorded from a lead II ECG for 30 minutes. Submaximal exercise tests were completed at 40% (low workload) and 70% (high workload) of peak VO2. Heart rate, VO2, RPE and blood lactate was recorded during steady state levels (4-6 minutes) at each stage. Data were statistically analyzed using paired t-test with significance set at p < 0.05. Following AMSD, baseline HRV was significantly reduced (118+10.2 vs. 103+8.7 ms). Significantly elevated POMS scores of tension, anger, and fatigue contributed to a significant increase in the total mood disturbance score (2.5+5.6 vs. 24.2+7.8). Exercise HR was significantly higher during both the low (122+2.8 vs. 132+2.5 bpm) and high (161+2.9 vs. 171+2.5 bpm) workloads following AMSD as compared to the rested state. Although blood lactate was significantly greater during the high workload following AMSD (7.5+0.7 vs. 9.4+0.7mmol/L) no significant differences were found between VO2, RPE, and blood lactate at any other workload following AMSD. These results indicate that AMSD elicits autonomic imbalance and impairments in both physiological and psychological function at rest and during exercise.